• Female

• New diagnosis of ductal carcinoma in situ (DCIS) and/or invasive breast carcinoma per histologic evaluation

• Age 45-79 at diagnosis

• Previous lumpectomy with surgical margins histologically free of invasive tumor and DCIS as determined by the pathologist.

• T stage of Tis, T1, or T2.

• T2 tumors must be ≤3 cm in maximum diameter

• If the tumor is human epidermal growth factor receptor 2 (HER2)-positive, the patient must receive HER2-directed therapy.

• For patients with invasive breast cancer, an axillary staging procedure must be performed (either sentinel node biopsy alone or axillary dissection \[with a minimum of 6 axillary nodes removed\]) and the axillary node\[s\] must be pathologically negative.

• Note: N0(i+) is not an exclusion criterion.

• Note: Patients meeting all of the following criteria are not required to undergo the axillary staging procedure:

‣ ≥70 years of age

⁃ estrogen receptor, Her2 = human epidermal growth factor receptor 2, PR = progesterone receptor negative (ER+PR+HER2-)

⁃ G1-2

⁃ Tumor ≤2 cm in size

⁃ Agrees to comply with aromatase inhibitor recommendation

• Ability to understand and the willingness to sign a written informed consent document in English